1. Field of the Invention
This invention relates to a digital fluoroscopy apparatus. In particular, the invention is an improvement in automatically correcting contrast levels of image data.
2. Description of Prior Art
In a digital fluoroscopy apparatus, a digital subtraction is usually carried out. In this technique, a fluoroscopic X-ray image of an anatomical region, such as the heart is made and converted to corresponding digital picture element (pixel) values. Shortly after one image is made, an X-ray contrast medium that has been injected into the blood vessels reaches the heart blood vessels, and then another X-ray image is made and converted to digital pixel values. When one of the images is subtracted from the other, digital difference pixel signals result. The difference signals are amplified and converted to analogue video signals which are fed to a television monitor that displays the difference image. In the difference image, some anatomical structures are deemphasized and a higher contrast and more easily visualized image of the mediuminfused vessels remains.
Methods and apparatus for performing a digital X-ray image subtraction are described in U.S. Pat. Nos. 4,204,225 and 4,204,226.
In a digital fluoroscopy apparatus, an important aspect for the image quality is to process a plurality of image signals while maintaining original contrast levels. That is, a precise diagnosis can be realized by processing the successive image signals to obtain the difference signals while keeping their precise contrast.
In the digital fluoroscopy apparatus, an analogue storage device, such as a video tape recorder (VTR) or a video disc recorder (VDR) is employed to store the analogue image signals which have been processed for an image diagnosis. When the analogue data signals that have been stored in the analogue storage device are read out and fed to the image processor after being analogue-to-digital converted, a variation occurs in the entire signal level (contrast level), or the peak values of the A/D-converted image data signals. This causes the following problem. For instance, firstly the mask image signal that has been acquired before the X-ray contrast medium reaches the interest region is stored in the VDR.
Secondly, the contrast image signal that has been acquired after the medium reaches it is stored in the video disc recorder. Then, the digital subtraction is performed between the mask image signal and the contrast image signal. A correct subtraction result may not be obtained because, as previously described, the signal levels of both the image signals vary from their original levels. These level variations are mainly caused by inherent characteristics of the amplifiers, coating fluctuations of the recording materials and reproducing characteristics of the magnetic head with respect to the analogue storage device, and conversion characteristics of the A/D and D/A converters.
Another prior art approach is to employ a digital magnetic disc memory instead of the analogue storage device, which is capable of storing directly the digital image signals that have been processed in the image processor. However, the total cost of such a digital magnetic disc memory is extremely more expensive than that of the analogue storage device.
An object of the present invention is to provide a digital fluoroscopy apparatus which automatically compensates for the level variations before the image data signals are processed.